1. Field of the Invention
The present invention relates to an inductor used in a radio frequency (RF) unit for a wireless communication terminal. More particularly, the present invention relates to an inductor having a low inductance value and a low inductance variation.
2. Description of the Related Art
Wireless communication terminals in the current marketplace are compact, light, consume a small amount of power, and have a low unit cost. Nevertheless, a further reduction in size, power consumption, and fabrication costs are primary considerations in manufacturing high-quality wireless communication terminals.
For this purpose, an RF port of a wireless communication terminal has to be made into one chip. Due to this requirement, it is critical to reduce the size of electronic devices constituting the RF port, in particular, the size of an inductor contained in a chip. In addition, maintaining the stability, as well as a high Q-factor, of an inductor is also important.
An inductor used in the RF port is mainly a spiral inductor or a bondwire inductor. As shown in FIG. 1, a conventional spiral inductor is formed by winding a metal wire several times on a plane. FIG. 1 includes an input port 10, an output port 12, and a mutual capacitance 16.
FIG. 2 is an equivalent circuit diagram of the conventional spiral inductor shown in FIG. 1. Referring to FIG. 2, a metal wire 18 is formed between the input port 10 and the output port 12 of the conventional inductor. A first parasitic capacitor 22 is positioned on the metal wire 18. Second parasitic capacitors 24 are interposed between the metal wire 18 and a substrate 20.
A self-resonant frequency (SRF) and a Q-factor of the conventional inductor decrease due to the first and second parasitic capacitors 22 and 24. In particular, if the substrate 20 is a silicon substrate, a signal input via the input port 10 leaks to the substrate 20, which increases the substrate loss.
A bondwire inductor is formed of a wire used in packaging bare chips (not shown) on the substrate, and has a high Q-factor and a low inductance. However, since a bondwire inductor occupies a large area when packaging the bare chips, the entire area of an RF chip increases. In addition, since the shape and length of the wire may gradually vary when processes of forming the bondwire inductor are performed, the inductance of the bondwire inductor may gradually vary.
In a case where an inductor having a low inductance is used, the effect of inductance variation may be large although the actual inductance variation is small. A low noise amplifier (LNA) of the wireless communication terminal exemplifies this situation. More particularly, the input impedance of the LNA in the wireless communication terminal depends on a source inductor of a first transistor and has to be reduced with an increase in a frequency. For example, a source inductor used in an LNA having a wireless LAN USII band of 5.725-5.825 GHz is primarily a bondwire inductor, a value of which is about 0.5 nH. In this case, the inductance of the source inductor may have an error of about 0.1 nH, which may be caused by a process variation. This error of about 0.1 nH, however, error degrades input reflection to 10 dB or more. This is a significant obstacle in designing LNA.